I would do a light grey grout. My one big rule is to never use a grout darker than the tile.

light on the fireplace, dark on the floor.

Try to do a grey grout that matches the tile. Don't you need a hearth by code?

Since your tile choice is intended to approximate wooden planks, my suggestion is that you use a grout color that is very similar, but ever so slightly lighter, than your tile color. The slightly lighter grout will let the tile be the main focus color.

"Don't you need a hearth by code?" - The IRC calls it a "hearth extension". It does not need to be raised, or a different color. If the finished flooring is noncombustible, it can also cover the hearth extension.

The SUPPORT of the finished material in the area of the required hearth extension must be noncombustible, and there can not be any combustible materials holding up the noncombustible material. But, the hearth extension does not have to be obvious when finished.

HOWEVER, This looks like a factory built fireplace ;-/

Which can be a little different per their listing.

Here is an example of what must be under the tile and on top of any combustible floor material in front of a specific factory built fireplace.

Hard to get all the info in one post on my phone... afraid of losing it.